A multi-link piston-crank mechanism for an internal combustion engine is known. This multi-link piston-crank mechanism includes: an upper link which is connected with a piston through a piston pin; a lower link which is rotatably attached to or mounted on a crank pin of a crank shaft and which is connected swingably with the upper link through a first connecting pin; a control link which includes a first end connected swingably with the lower link through a second connecting pin; and a control shaft which is rotatably attached to or supported by a cylinder block and which is provided with an eccentric shaft supporting a second end of the control link swingably. This multi-link piston-crank mechanism is arranged so that the first connecting pin is located on one side of a cylinder center axial line and the second connecting pin is located on the other side of the cylinder center axial line.
In this multi-link piston-crank mechanism, a force is applied to the piston in the direction pressing the piston to the cylinder inside wall surface on the side on which the control link lies as viewed in the axial direction of the crank shaft, owing to the construction of this mechanism.
For a multi-link piston-crank mechanism of the above-mentioned type, a patent document 1 discloses an arrangement including an oil supply hole having a forward open end opening in a thrust bearing surface around the crank pin of the crank shaft, and an oil guide groove which is formed in an annular flange surface of the lower link sliding on the thrust bearing surface, and which extends in the radial direction of the crank pin and aligns with the forward open end of the oil supply hole at a predetermined swing posture of the lower link. With this arrangement, a lubricating oil (oil jet) is supplied to the inside wall surface of the cylinder from the lower link along the oil guide groove.
However, in the arrangement, as an example, in which the center of the crank pin is always set off largely to the other side with respect to the cylinder center axial line in the view in the crank shaft axial direction, the crank pin is located toward the other side beyond the cylinder inside wall surface on the control link's side. Therefore, in the above-mentioned multi-link piston-crank mechanism, it is not possible in some cases to set the direction of the oil guide groove extending in the radial direction of the crank pin toward the cylinder inside wall surface on the control link's side without regard to the swing posture of the lower link.